This invention is related to pigment dispersant that is useful in a cationic electrocoating composition.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in an electrocoating bath of an aqueous emulsion of film-forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in a bath of a typical cathodic electrodeposition process also are well known in the art. These resins are typically made from polyepoxide resins which have been chain extended and then an adduct is formed to include amine groups in the resin. Amine groups typically are introduced through a reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode The article to be coated is the cathode and is passed through the tank containing the electrocoating bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The resulting coated article is removed from the bath after a set period of time and is rinsed with deionized water. The coating on the article is cured typically in an oven at sufficient temperature to produce a crosslinked finish on the article.
Cathodic electrocoating compositions and resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek et al U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. Pat. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. Pat. No. 4,137,140 issued Jan. 30, 1979 and Wilmer et al U.S. Pat. No. 4,468,307 issued Aug. 25, 1984.
Pigments are a necessary component in a typical eletrocoating automotive primer composition. Pigment dispersants are used to disperse the pigment in the composition and keep the pigment dispersed in the composition and thus are a very important part of any electrocoating composition. In the dispersion process, primary pigment particles are separated from agglomerates or aggregates of these particles; accluded air and absorbed water are displaced and the the surface of the pigment is coated with the pigment dispersant. Ideally, each primary particle which has been mechanically separated during the dispersion process, also is stabilized against flocculation. If the pigment particles are not properly dispersed and stabilized in the composition, the advantages built into the pigment by the manufacturer may be lost. For example, the pigment may settle in the electrodeposition bath which can result in loss of corrosion protection of the substrate. In addition, appearance of a film deposited by the electrodeposition process and the operating characteristics of an electrocoating bath may be adversely affected by inadequate pigment dispersion.
The better the pigment dispersant used in a coating composition or electrocoating bath, the less dispersant is required and the pigment to binder ratio can be increased in the composition. This can result in a savings on dispersant costs, improved processability, increased production capacity, and lower VOC (volatile organic concentration) in the coating or electrodeposition bath. Current commercial pigment dispersants used in cathodic electrocoating processes typically are polyepoxide resins containing either onium salts or amine salts. Using the pigment dispersants known in the prior art, the maximum pigment to binder ratio that can be obtained is 3:1. These pigment dispersants also require the use of solvents which raise the VOC of the electrocoating bath. Current commercial pigment dispersants contain at least 30 to 40% solvent.
What is needed is a pigment dispersant which will allow a maximum pigment to binder ratio, and minimize or eliminate the amount of solvent used in a typical electrocoating bath. This will result in a cost savings of dispersant, improved processability, and a lower VOC of the electrocoating bath.